There's a misnomer commonly used in cosmology: "Recombination Era". That term refers to the time, about half a million years or so after the Big Bang, when the expanding universe cooled down enough for atoms to form. As the temperature dropped below a few thousand degrees electrons were finally able to attach themselves to nuclei; before that they roamed freely in a plasma. Free-electron plasmas are quite good at scattering light. It's impossible to see very far through them.

Suddenly, at the Recombination Era the cosmos became transparent. Photons decoupled from matter. The fog lifted.

But a linguistic quibble remains: Re in Recombination means "again" --- but there was never any "combination" before that time. Yet nobody ever says "Combination Era"....